R help - May 2009 - matching period with perl regular expression

Hello,

I have several strings where I am trying to eliminate the period and
everything after the period, using a regular expression. However, I am
having trouble getting this to work.
> x = "wa.w"
> gsub(x, "\..*", "", perl=TRUE)
[1] ""
Warning messages:
1: '\.' is an unrecognized escape in a character string
2: unrecognized escape removed from "\..*"

In perl, you can match a single period with \.
Is this not so even with perl=TRUE. I would like for x to be equal
to
> x = "wa"
What am I missing here?
-stephen
=========================================Stephen J. Barr
University of Washington
WEB: www.econsteve.com

Try this:

gsub("^(\\w*).*$", "\\1", x)

On Wed, May 13, 2009 at 8:41 PM, Stephen J. Barr
<stephenjbarr@gmail.com>wrote:
> Hello,
>
> I have several strings where I am trying to eliminate the period and
> everything after the period, using a regular expression. However, I am
> having trouble getting this to work.
>
> > x = "wa.w"
> > gsub(x, "\..*", "", perl=TRUE)
> [1] ""
> Warning messages:
> 1: '\.' is an unrecognized escape in a character string
> 2: unrecognized escape removed from "\..*"
>
> In perl, you can match a single period with \.
> Is this not so even with perl=TRUE. I would like for x to be equal to
> > x = "wa"
>
> What am I missing here?
> -stephen
> =========================================> Stephen J. Barr
> University of Washington
> WEB: www.econsteve.com
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

	[[alternative HTML version deleted]]

R interprets backslash to give special meaning to the next character, i.e.
it strips off the backslash and send the following character to gsub
possibly reinterpreting it specially (for example \n is newline).  Thus
a backslash will never get to gsub unless you use a double backslash.
Thus we can use "'\\." to represent \.   Also note that
that the regular expression "[.]" represents a literal dot and does
not
require a backslash in the first place.  You don't need perl  = TRUE for
simple regular expressions like this.

On Wed, May 13, 2009 at 7:41 PM, Stephen J. Barr <stephenjbarr at
gmail.com> wrote:
> Hello,
>
> I have several strings where I am trying to eliminate the period and
> everything after the period, using a regular expression. However, I am
> having trouble getting this to work.
>
>> x = "wa.w"
>> gsub(x, "\..*", "", perl=TRUE)
> [1] ""
> Warning messages:
> 1: '\.' is an unrecognized escape in a character string
> 2: unrecognized escape removed from "\..*"
>
> In perl, you can match a single period with \.
> Is this not so even with perl=TRUE. I would like for x to be equal to
>> x = "wa"
>
> What am I missing here?
> -stephen
> =========================================> Stephen J. Barr
> University of Washington
> WEB: www.econsteve.com
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

You have the arguments out of line and you need two backslashes:
> x <- "wa.w"
> gsub("\\..*", "", x)
[1] "wa"
>  

Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Stephen J. Barr
Sent: Thursday, 14 May 2009 9:42 AM
To: r-help at r-project.org
Subject: [R] matching period with perl regular expression

Hello,

I have several strings where I am trying to eliminate the period and
everything after the period, using a regular expression. However, I am
having trouble getting this to work.
> x = "wa.w"
> gsub(x, "\..*", "", perl=TRUE)
[1] ""
Warning messages:
1: '\.' is an unrecognized escape in a character string
2: unrecognized escape removed from "\..*"

In perl, you can match a single period with \.
Is this not so even with perl=TRUE. I would like for x to be equal
to
> x = "wa"
What am I missing here?
-stephen
=========================================Stephen J. Barr
University of Washington
WEB: www.econsteve.com

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Gabor Grothendieck <ggrothendieck <at> gmail.com> writes:
> 
> R interprets backslash to give special meaning to the next character, i.e.
> it strips off the backslash and send the following character to gsub
> possibly reinterpreting it specially (for example \n is newline).  Thus
> a backslash will never get to gsub unless you use a double backslash.
To quote Peter Dalgaard:

"The generic rule for backslashes is that you need twice as many as you
thought"

Only make sure that you have the right stopping rule for recursive evaluation of
that.

Dieter